DESCRIPTION There is a great interest in imaging neuronal activity based on changes in fast intrinsic optical signals (e.g. changes in light scattering and phase) that occur on a millisecond timescale. Fast intrinsic optical signals are related to alteration in the complex refractive index and small volume changes near the neuron membrane, in response to the rapid osmotic changes associated with ion fluxes during action potentials. Optical coherence tomography (OCT) is an emerging biomedical imaging technology that provides label-free and depth-resolved images with micron-scale spatial resolution and sub-millisecond temporal resolution. OCT relies on detection of intrinsic optical contrast, eliminating the need for potentially toxic exogenous contrast agents or genetically- encoded indicators. OCT achieves over 100 dB sensitivity, enabling it to detect weak scattering changes associated with neuronal activity. In addition, OCT has extremely good phase sensitivity (<one millirad, corresponding to an optical path length change of ~100 pm), making it possible to detect nanometer-scale membrane displacement associated with neural action potentials with over 10x signal-to-noise ratio. Recently, our group developed a space-division multiplexing OCT (SDM-OCT) technology which allows parallel and synchronized imaging from multiple sample locations. In this exploratory program, we plan to evaluate the feasibility of using high resolution (<3 ?m) and ultrahigh speed (3.2M A-scans/s) SDM-OCT technology to image and characterize activity-dependent fast intrinsic optical signal changes, especially phase changes. Validation experiments will be conducted using in vitro rat 2D neural cultures and 3D organotypic hippocampal cultures to correlate fast intrinsic optical signal changes imaged with SDM-OCT with electrophysiological stimulation and recording of neural activities. Since each vertical cross sectional OCT image will contain hundreds of neurons, intrinsic optical signals from thousands of neurons can be recorded simultaneously using the space-division multiplexing technology. If successful, this label-free imaging technology can become a powerful platform to investigate behavior of thousands of neurons in a network simultaneously, with the potential to significantly impact fundamental brain research.